Various techniques have been used for non-contact sensing of target materials. One class of existing techniques can be referred to as reflectance spectroscopy. Infrared (IR) reflectance spectroscopy directly detects back-scattered radiation at the pump wavelength, which can also be used to determine optical properties of the target material surface.
A second class of techniques that probe infrared absorption of target materials are alternative photothermal approaches that involve measurement of changes in the thermal emission of the surface (measured using infrared radiometry) as a function of incident wavelength. These techniques can measure either the total change in thermally radiated power associated with the temperature rise of the surface or the change in the spectrum of the blackbody emission, modulated by the characteristic emissivity of the surface material.